In a conventional four-stroke internal combustion engine, each piston is driven by the ignition of the fuel charge during only one stroke of its four-stroke cycle. This intermittent application of driving power normally requires the employment of a mechanical flywheel to smooth out the cyclic variation in torque applied to the main drive shaft. The present invention is specifically designed to minimize the torque variation referred to by generating a piston driving force which can be applied to the pistons during any or all of the various strokes of its operating cycle. Although the invention described primarily in connection with its employment in an internal combustion engine operating on a four-stroke cycle, it will be readily apparent that a principle described is equally applicable to engines operating on a two-stroke cycle or to rotary piston engines.